1. Field of Invention
The invention relates to a method for controlling the stretching of rolling stock, particularly of pipes, by way of a pilot control system for the rolling mill, taking into consideration the measured physical values of the rolling stock.
2. Description of the Prior Art
The primary objective of using measurement and control techniques in rolling mill installations is to increase the dimensional accuracy and quality of rolling mill products. The flexibility of a production plant can also be increased by automation measures, in order to manufacture small lot sizes of rolled products economically. The control system generally consists of a pilot control system, by means of which the reshaping of the rolling stock is planned in advance, and a control component, which consists of a feedback of measurement data of the rolling stock in order to adjust just the rolling mill. In comparison with the control system, the pilot control system is adjusted more quickly and, in principle, controls the process more extensively.
If it is possible to make the pilot control system as accurate as possible and to determine the actuating variables as extensively as possible, then the first product of the series of a rolling milling production can already be produced with the high dimensional accuracy that is required. Variations in the actuating variables, such as in the dimensional changes in the initial product and in the temperature losses, can be timely detected and compensated for by means of a pilot control system of the rolling mill.
It is well known that the dimensional accuracy of the rolling stock can be prejudiced by its properties, particularly by its dimensions, by the composition of its material and by its temperature. The dimensions of the initial product can be determined by measurement, for example, with the help of isotope or eddy current measurement methods. These methods can be used with sufficient accuracy. The material composition usually is known from the production planning by following the material.
Radiation pyrometers, with which the temperature of the surface of the rolling stock can be determined optically, are usually used to measure the temperature of the rolling stock. Frequently, however, the temperature measurement data is not suitable for use in the control systems, since such data can deviate considerably in magnitude as well as in trend from the effective average temperature of the rolling stock. For control systems, however, only one measured value can be taken as suitable. This measured value must represent the average temperature in the cross section of the rolling stock since only this parameter permits determination of the change in the flow behavior of the material.
The process-induced effects, which call cause errors when a radiation pyrometer is used and distort the result of the measurement, are cinder particles or an inhomogeneous temperature distribution in the cross section of the rolling stock.
Different temperature gradients, even temperature gradients of unknown algebraic sign, can arise in the material depending on the prior history of the rolling stock (heating, re-shaping, cooling). The heat is introduced into the rolling stock and is dissipated once again over the surface, so that especially here a particularly high temperature gradient can arise.
It follows from all the foregoing that considerable errors must necessarily arise if the average temperature in the cross section, which alone has a determining effect on the stretching of the rolling stock, is estimated from the surface temperature. This is particularly serious because a wrong value, used in the pilot control system, can lead to larger errors than a nominal value.